This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of aqueous surfactant solutions in which polyalkylene oxide sacrificial agents are employed to reduce surfactant adsorption.
In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding techniques. To date one of the more promising low tension waterflooding techniques involves the injection of aqueous solutions of petroleum sulfonates within a designated equivalent weight range and under controlled conditions of salinity. For example, in a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", JOURNAL OF PETROLEUM TECHNOLOGY, Vol. 25, February, 1973, pp. 205-210, there is disclosed a low tension waterflood process which involves the sequential injection of a protective slug, a surfactant slug, and a mobility control slug. The protective slug is an aqueous solution of sodium chloride which is injected in order to displace the reservoir waters ahead of the subsequently injected surfactant slug. This slug is substantially free of divalent ions which would tend to precipitate the subsequently injected surfactant. The rear portion of the protective slug normally contains an inorganic sacrificial agent such as sodium tripolyphosphate and/or sodium carbonate which acts to satisfy adsorption sites within the reservoir, thus decreasing absorption of the surfactant.
The surfactant slug comprises an aqueous solution of petroleum sulfonates exhibiting an average equivalent weight within the range of 350-500 and which contains sodium chloride in a concentration, typically about 1.0 to 2.0 weight percent, which will promote the desired low interfacial tension between the injected water and the reservoir oil. The surfactant slug also will normally contain a sacrificial agent such as described above. The subsequently injected thickened water slug contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide an initial viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. Thereafter a driving fluid such as produced field brine is injected in order to carry the process to conclusion.
As noted previously, one limitation encountered in waterflooding with petroleum sulfonates, as well as with many other anionic surfactants, lies in the tendency of the sulfonates to precipitate from solution in the presence of even moderate concentrations of divalent metal ions such as calcium and magnesium ions. Normally it is considered that the aqueous surfactant solution should contain no more than perhaps 50 to 100 parts per million of such divalent metal ions. Another limiting factor present in the use of such surfactants is the fact that the desired low interfacial tensions can seldom be achieved, even in the absence of divalent metal ions, at salinities in excess of about 2 to 3 weight percent.
Many subterranean oil reservoirs contain formation waters having divalent metal ion concentrations and monovalent salt salinities well in excess of the values which can be tolerated by the petroleum sulfonate type surfactant systems. Similar considerations apply in many cases to the waters which may be available for injection purposes. Thus considerable effort has been devoted to the development of surfactant systems which may be employed in environments containing relatively high divalent metal ion concentrations and/or relatively high monovalent salt salinities. For example, U.S. Pat. No. 3,508,612 to Reisberg et al. is directed to a waterflooding process employing a calcium compatible surfactant system which is comprised of dissimilar anionic surfactants. A typical anionic surfactant system disclosed by Reisberg et al. comprises a petroleum sulfonate having a molecular weight within the range of 430-470 and a sulfated oxylated alcohol containing a C.sub.12 -C.sub.15 alkyl group and from 3-9 ethylene oxide groups.
U.S. Pat. No. 3,792,731 to Feuerbacher et al. discloses an anionic-nonionic surfactant system for use in low tension waterflooding where the monovalent salt concentration is within the range of 0.5 percent to about 15.0 percent. Among the anionic surfactants disclosed in Feuerbacher et al. are petroleum sulfonates, alkyl sulfonates and sulfates, and sulfosuccinates. The various nonionic surfactants suggested by the patentees include the polyalkylene oxides such as polymers and copolymers of ethylene oxide and/or propylene oxide as well as ethoxylated thioethers, ethoxylated amines and ethoxylated alcohols and alkyl phenols.
Yet another anionic-nonionic surfactant system for use in low tension waterflooding is disclosed in U.S. Pat. No. 3,811,504 to Flournoy et al. This patent is directed to a three-component system containing two anionic surfactants and one nonionic surfactant which is said to be usable in environments exhibiting polyvalent ion concentrations of about 1,500 to 12,000 parts per million. In this procedure, one of the anionic surfactants is an alkyl or alkyl aryl sulfonate and the other is an alkyl polyethoxy sulfate. The nonionic surfactant includes polyethoxylated alkyl phenols in which the alkyl group has 5 to 20 carbon atoms with 6 to 20 polyethylene oxide groups and polyethoxylated aliphatic alcohols with an alkyl chain having 5 to 20 carbon atoms and again containing 6 to 20 polyethylene oxide groups. The Flournoy et al. patent also discloses that the surfactant slug may be preceded by sacrificial inorganic materials such as sodium carbonate or sodium polyphosphate.
Yet a further procedure for low tension waterflooding in the presence of high divalent and/or monovalent salt concentrations is disclosed in U.S. patent application Ser. No. 560,289, filed Mar. 20, 1975, by Silvia C. Birk. This procedure involves the injection of a multifunctional surfactant system comprising a surface-active amide-linked sulfonate anionic group and a surface-active polyethylene oxide nonionic group. Among the surfactants which may be employed in this process are disubstituted taurines such as sodium N-methyl-N-oleoyl-taurate available from General Aniline and Film Corporation under the trade name "Igepon T-33" and polyethoxylated alkyl phenols such as the polyethoxylated nonyl phenol available from General Aniline and Film Corporation under the trade name "Igepal CO-520".